Recent advances in facsimile devices, including printers therefor, have been remarkable. In these devices, a heat-sensitive recording system is widely used that utilizes a combination of a thermal head and a heat-sensitive recording paper having a coating of a colorless dye such as crystal violet lactone and a phenol compound, as described, for example, in Japanese Patent Publication No. 14039/70 (corresponding to U.S. Pat. No. 3,539,375).
The heat-sensitive recording system has many advantages. For example, since the recording paper is of the primary color formation type, a conventional development step is not needed, and the recording unit can be relatively simple. Therefore, the production costs for the recording paper and recording unit are low. Also, heat-sensitive recording is of the non-impact type and is not noisy. Thus, heat-sensitive recording has gained a position as a low speed recording system.
However, a major disadvantage of the heat-sensitive recording system is that it is low in recording speed compared with other recording systems, such as electrostatic recording. For this reason, the heat-sensitive recording system has not yet been successfully employed in circumstances where very high speed recording is required.
The main reason for which high speed recording cannot be attained by the heat-sensitive recording system is that heat conduction cannot be achieved sufficiently quickly between the thermal head and the heat-sensitive recording paper coming into contact with the thermal head, and, thus, sufficient recording density cannot be obtained if high speed recording is attempted. The thermal head, comprising electric resistor heat generators combined together in a dot matrix form, generates heat upon receipt of a recording signal and melts a heat-sensitive color-forming layer in contact with the thermal head, thereby allowing it to form color. In order to attain recording image of high sharpness and high density, it is necessary that dot reproduction be good. That is, it is necessary that the thermal head and the heat-sensitive color-forming layer come into as close contact as possible and heat conduction be carried out with high efficiency so that a fully colored dot conforming to the shape of the dot heat generator of the thermal head will be formed in the heat-sensitive color-forming layer in a manner completely correspondent to a high speed recording signal. In fact, however, only several percent of the amount of heat generated from the thermal head is conducted to the heat-sensitive color-forming layer and the efficiency of heat conduction is extremely low. Several methods have been proposed to increase the smoothness of the heat-sensitive color-forming layer so that the thermal head and the heat-sensitive color-forming layer will come into as close contact as possible with each other.
Japanese Patent Publication No. 20142/77 described a method in which the surface of the heat-sensitive color-forming layer is treated so that the surface smoothness as represented in terms of Bekk smoothness is from 200 to 1,000 seconds. Japanese Patent Application (OPI) No. 115255/79 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") describes that when the Bekk smoothness is from 200 to 1,000 seconds, the heat-sensitive color-forming layer can respond only to heat impulses of from about 5 to 6 milliseconds, and that for high-speed recording using heat impulses of less than 1 millisecond it is necessary for the surface of the heat-sensitive color-forming layer to be made smooth to such an extent that the Bekk smoothness is more than 1,100 seconds. However, when the Bekk smoothness is increased to more than 1,100 seconds, color fog is produced upon application of pressure. The formation of color fog is prevented by using a base paper which has been previously made smooth to an extent that the Bekk smoothness is more than 500 seconds. Japanese Patent Application (OPI) No. 156086/78 describes that the surface roughness, Ra, of the heat-sensitive color-forming layer is made to be less than 1.2 .mu.m, and the gloss to less than 25%.
In all of the above-described prior art techniques, the smoothness of the heat-sensitive color-forming layer is increased solely by calendering operations such as super-calendering, machine calendering, and gloss calendering. This calendering operation is applied to the base paper alone, or to the base paper and the heat-sensitive paper, or to the heat-sensitive paper alone. In the heat-sensitive recording paper, however, as its smoothness is increased by calendering in order to provide an increased recording density, the chance of sticking or piling occurring is increased. In practical use, therefore, the smoothness of recording paper is suppressed to a suitable level so that the recording density and the occurrence of sticking or piling are properly balanced. In the prior art techniques, whichever level of smoothness is selected, the resulting heat-sensitive recording paper has been unsuitable for practical use for high-speed recording with respect to recording density and recording stability.
The term "sticking" as used herein refers to a phenomenon wherein the thermal head adheres to the heat-sensitive color-forming layer, thereby producing stripping noise and lowering the dot reproduction. The term "piling" refers to a phenomenon wherein heat-melted products of the heat-sensitive color-forming layer accumulate on the thermal head, thereby lowering the recording density and dot reproduction. Both of these phenomena will inhibit stable recording.
Another disadvantage as encountered in applying the calender treatment to the heat-sensitive recording paper is that color fog is formed by pressure, resulting in an increase in the density of the background of the recording paper. Similarly, in the calendering of the base paper, so-called cockle, wrinkles, etc., due to unevenness in basis weight will develop. Thus, the calendering operation is limited in its practical use. As described above, the attempt to increase the smoothness of the heat-sensitive color-forming layer by calendering so as to increase the recording density has met with only limited success, and the resulting heat-sensitive recording paper is not sufficiently satisfactory for use in high-speed recording.
Furthermore, heat-sensitive recording paper is usually used in a roll form in which it is wound on a paper tube several inches in diameter, and curling is liable to occur on the roll as a result of prolonged use. It has long been desired to overcome this curling problem.